Belt drives are used extensively to transmit power from a motor to rotating equipment or machinery. In the simplest example, a flat belt is wrapped around a pulley on a motor and a pulley on a shaft. The belt must have tension in order to transmit power via frictional forces from one pulley to the other pulley. The belt can slip if it is too loose for the amount of power being transmitted.
Toothed belt drives are much less susceptible to slippage. In a toothed belt drive, the pulleys and belt have teeth that engage to transmit more power with less belt tension. However, belt tension is still a necessity.
In the field of machine tool spindles, one of the most common methods of belt tensioning is to increase the center distance between the spindle shaft and the motor. More particularly, according to the method, one end of a threaded rod is fixed to a motor unit while the other end is pulled upon by a nut and bracket that is directly or indirectly attached to a spindle unit. An operator tightens the nut and thereby induces tension on the belt. Once the belt is tensioned, the motor unit is clamped in place.
The problem with this method is that operators who install or replace belts tend to over-tighten the nut and ultimately create excessive belt tension. Excessive belt tension is a major cause of premature spindle failure, which typically involve bent shafts, overloaded bearings, and the need to frequently change belts.
At first look, one might think that this is simply a matter of discipline, and that if a mechanic is provided a torque wrench and a torque value at which to turn the nut, then over tightening should never happen. With this approach, it would theoretically be possible to build some type of device that assures use of the torque wrench in this manner. To some degree this approach is valid, but it also has drawbacks.
For instance, some may believe that reaching a given torque value is all that is necessary to apply a related linear force where it is desired, and thereby move or secure objects in an expected manner. A predictable situation like this is often present on a factory assembly line where every piece is new, well lubricated, without defect, and installed with adequate access and visibility. However experienced field service mechanics know there are all kind of situations encountered in the field. For example there may be rusty or damaged threads, bowed rods, foreign material blocking smooth movement, and lack of space to use a torque wrench correctly. Many mechanics will not rely solely on a torque wrench, but will sometimes use other ways to check the results of their work. For example they may look closely at the mated parts, and perhaps perform physical measurements.
There are always times when mechanics may not have access, time, or knowledge to perform a check. In such cases they may install a belt too tightly or too loosely. Since a loose belt will be quickly apparent and troublesome and call for additional effort, a mechanic may choose to err on the side of too much belt tension. Further, if after a period of time a belt is slipping due to conditions such as wear or deterioration, some mechanics may choose to over tighten the belt rather than replace it. This is often driven by short term expediency at the expense of long-term spindle health and the related costs.
For these reasons and others, it is desirable to facilitate the proper tensioning of a new belt, and also the adjustment of an installed belt. Such a device should be relatively easy to use for tightening. When it is necessary to remove an old belt, such a device should not interfere with the mechanic's ability to use high amounts of loosening torque to overcome damage and other things that have happened during the time since the belt was installed.